Silk has been used as a textile material for over 4000 years. Due to its high (tensile) strength, luster, and ability to bind chemical dyes, silk remains the premier textile material in the world today.
Naturally occurring silk is produced by insects and spiders. Most commercially produced silk is harvested from cocoons of Bombyx larva, or silkworms (B. mori ). Unlike the silkworm, which produces silk only for use in building its cocoon, spiders produce a variety of different silks throughout their adult life. Although all spiders produce silk, the specialized use of silk is most developed in the orb-weaving spiders. The golden orb weaver spider, Nephila clavipes (N. clavipes), is one of the most carefully studied orb-weavers with respect to the production and properties of their silk. The orb-web weaving spiders produce a broad range of high-performance structural fibers with mechanical properties that are superbly matched to their function. These spiders produce seven different silks in various glands that are stored in the liquid state, and each is used to make silk for a specific purpose.
In particular, the Golden Orb Weaver spider constructs its dragline and its web frame threads using silk from the major ampullate gland. This so-called dragline silk has an unusual combination of high mechanical strength and elasticity because it must provide support for the web as well as allow significant web deformation without breaking when the spider's prey are caught. The strength and elasticity of the silk are also exhibited in its use as a dragline, which supports the spider's weight on a single thread and resists breaking when the spider falls. This desirable combination of strength and elasticity, as well as its other extraordinary mechanical properties, makes dragline silk a potentially useful commercial material.
Silk fiber in general exhibits mechanical properties similar or superior to other fibers. A few synthetic polymers such as Kevlar.RTM. have a slightly higher strength than Nephila clavipes dragline, but their toughness is significantly lower. The mechanical properties of dragline silk fibers are in general superior to those of B. mori silk fibers. The excellent mechanical properties of dragline silk indicate that it may be desirable to use in fiber-reinforced composite materials. Dragline silk fibers are stronger per unit weight than high-tensile steel and have tensile strength approaching that of aramid fibers. Dragline silk is exceptionally tough and can stretch to about one hundred and thirty percent, and absorb a tremendous amount of energy before failure.
Instead, the prior art teaches that polypeptides such as naturally occurring silkworm cocoon silk fiber can be dissolved under specific conditions followed by fiber spinning using any of several well-known methods. For example, U.S. Pat. No. 5,171,505 teaches the dissolution of natural or synthetic polypeptides in hexafluoropropanol or a formic acid/lithium halide mixture, followed by conventional wet, dry, or dry-jet wet spinning. Lock U.S. Pat. No. 5,252,285 teaches a method to spin fibers from cocoon silk. Noting that cocoon silk in its native fiber form is insoluble in hexafluoropropanol, Lock '285 teaches a pretreatment of dissolving cocoon silk in an aqueous salt solution, followed by dialysis to remove the salt and drying to remove the water; Lock '285 teaches that after this pretreatment, the cocoon silk is dissolved in hexafluoropropanol followed by fiber spinning by conventional wet, dry, or dry-jet wet spinning. Also, U.S. Pat. No. 5,252,277 teaches a method to spin polypeptides fibers from a solution of polypeptide in a liquified phenol and lithium thiocyanate.
A particular type of fiber reinforced composite material is the so-called nano composites. To be utilized as reinforcement in nanocomposites, fibers should be in the range of about 1 nm to about 1000 nm. Conventional techniques used to spin fibers from solution such as wet spinning, dry-jet wet spinning, and dry spinning produce fibers in the range of 10 to 100 microns. It is difficult to make nanometer-range diameter fibers using conventional spinning processes. In contrast, electrospinning is well suited to producing fibers with nanometer-range diameters. The diameter of electrospun fibers is typically one to two orders of magnitude smaller than the diameter of conventionally spun fibers. The use of electrospinning is well known in the art.
Another useful application for fibers in the nanometer size range is in materials characterization rising transmission electron microscopy (TEM) and electron diffraction (ED). Characterization of several physical properties of a material including but not limited to surface features and sample geometry (by Transmission electron microscopy) and crystalline content (by Electron diffraction) are facilitated using fibers in the size range of about 1 nm to about 5,000 nm.
Although the prior art teaches methods to produce fibers from polypeptides or silkworm silk, the prior art does not teach a method to spin nanofibers from spider dragline silk. Because dragline silk nanofibers are desirable as a reinforcement in nanocomposite materials as well as in other applications. A need exists, for a method to produce such fibers.